The present invention relates to transistor amplifier, and more particularly, to a stabilization circuit for a Class C transistor power amplifier of the type conventionally used in radio communications apparatus.
A Class C transistor amplifier is commonly defined as a transistor which is operative, i.e., conducting, for less than one-half of a cycle in the input signal to the transistor. For the remaining portion of the input signal cycle, the transistor remains in a cutoff condition and a very small value of cutoff current flows from the transistor due to the biasing of the transistor. However, when the input signal to the transistor amplifier lies within the radio frequency range, i.e. 10 KHz--100 GHz, the conventional definition of a Class C transistor amplifier in terms of output characteristics may not be entirely accurate. For example, when operating in the upper end of the radio frequency range, the response characteristics of the transistor amplifier may be such that the transistor remains in a conducting state throughout the entire cycle of the input signal. However, if the input signal to the transistor has a sufficiently low frequency, the output signal from the transistor amplifier will conform to the functional definition given previously.
Therefore, the term "Class C transistor amplifier", as used throughout the present specification, refers to the bias placed on the transistor rather than the output characteristics thereof. In other words, the Class C transistor amplifier to which the present invention pertains has a bias such that the transistor is conducting for less than one-half of the cycle of the input signal when the signal has a sufficiently low frequency, but may conduct for more than one-half of the input cycle at higher frequencies.
Since a Class C transistor amplifier utilizes a relatively sensitive transistor, one problem inherent to these amplifiers is the presence of spurious oscillations in the output signal of the amplifier. These oscillations can be caused by an input signal having a particular frequency, or by changes in the amplitude of the input signal or the load which is placed on the transistor.
In order to reduce to the inherent tendency of the transistor amplifier to oscillate, a solution has been proposed in which the transistor base circuit is connected to ground through a relatively large value resistor. This large value resistor substantially inhibits the tendency of the amplifier to oscillate and therefore appears to solve the problem. However, this method of inhibiting oscillations can only be used when the amplifier is operating under normal anticipated load. If the load changes from the optimum point, for example during tuning of the circuit in which the amplifier is located or inadvertent disconnection of an antenna from the circuit, high voltages can be developed in the transistor collector tuned circuit which may result in a voltage breakdown of the collector to base junction of the transistor. This collector to base breakdown has no serious consequences in a conventional amplifier circuit since the current resulting from the breakdown can be shunted to ground through an RF choke and/or a low value resistor. However, if the base circuit is connected to ground through a relatively large value resistor in order to obtain the stabilizing effect and thus inhibit oscillation, the collector-base breakdown will heavily forward bias the transistor which will in most cases result in destructive failure of the transistor.
The use of a transistor having a higher breakdown voltage does not solve this problem because there always exists some load variation which will cause a failure of the transistor. Furthermore, the operational characteristics of such a high breakdown transistor may not be correct from a design standpoint for the desired operation of the normal load, and therefore such a transistor will not be satisfactory. At present, there is no transistor available which is immune to all possible variations in the load and which operates in the proper fashion.